The quality of Bed and Breakfast, named ”experience goods”, is only learned by the consumers after usage. To get the quality certification not only improve the quality of B&B products but also reveal the information of quality to consumers. However, it is common for consumers to believe that the more firms joined in the quality certification, the more reliable is the information provided by it. Moreover, larger scale of quality certification might attract more firms to join it. This might form a network-externality like structure. This thesis investigates the equilibrium number of firm join the quality certification and its impact on social welfare.